Many formed products of metal are produced by forming respective portions and then bonding the portions together by welding or the like in a later process. Meanwhile, some methods have been proposed to simultaneously perform hot forming and diffusion bonding by utilizing heat in the hot forming in order to reduce the number of processing steps or to obtain a composite formed product that cannot be produced by the method mentioned above.
For example, Patent Literature 1 proposes a method in which a metal member and an insert material are placed in a forging die with a temperature at which the metal member and the insert material become semi-molten to simultaneously perform forging and bonding, thereby obtaining an intermediate product, and then, another metal member and an insert material of the intermediate product are forged and bonded together at a temperature at which the other metal member and the insert material become semi-molten, as in the previous process, thereby obtaining a final product.
In addition, Patent Literature 2 proposes a method for performing blow forming and diffusion bonding by utilizing heating in the blow forming process. Blow forming is one of hot working processes for aluminum and used mainly in thin-plate processing. Some metals consisting of fine crystalline grains are known to cause superplastic deformation when deformed in a high temperature range and exhibit large extension of 100% or more. Blow forming is a forming method that utilizes the phenomenon, in which a metal plate member is pinched between dies heated to high temperature, then heated, and formed into a shape of the die by a high pressure gas such as nitrogen gas. Blow forming is advantageous in that when a material having favorable superplastic deformability is used for a metal plate member, cold press forming allows integral forming for a part whose shape is so complicated that forming thereof is extremely difficult. In Patent Literature 2, a plurality of plates are stacked on each other; then, regions of the plates desired to be bonded are heated and pressed by a blow forming die to be subjected to diffusion bonding, and, simultaneously, gas pressure is applied inside the blow forming die to perform blow forming, thereby forming a part with a hollow structure.